This invention relates to ink-jet printheads, and more particularly to asymmetric fluidic techniques for the printheads.
The present invention is generally related to a printhead for an inkjet printer and more particularly related to the design of ink feed channels and ink firing chambers within the printhead.
Thermal inkjet printers operate by expelling a small volume of ink through a plurality of small nozzles or orifices in a surface held in proximity to a medium upon which marks or printing is to be placed. These nozzles are arranged in a fashion in the surface such that the expulsion of a droplet of ink from a determined number of nozzles relative to a particular position of the medium results in the production of a portion of a desired character or image. Controlled repositioning of the substrate or the medium and another expulsion of ink droplets continues the production of more pixels of the desired character or image. Inks of selected colors may be coupled to individual arrangements of nozzles so that selected firing of the orifices can produce a multicolored image by the inkjet printer.
Speed of printing (droplet ejection rate) and quality of print are essential to the user of an inkjet printer. Other factors such as spurious ink spray reduction and accurate positioning of the drop on the medium are also important.
Expulsion of the ink droplet in a conventional thermal inkjet printer is a result of rapid thermal heating of the ink to a temperature which exceeds the boiling point of the ink solvent and creates a vapor phase bubble of ink. Rapid heating of the ink can be achieved by passing a square pulse of electric current through a resistor, typically for 0.5 to 5 microseconds. Each nozzle is coupled to a small unique ink firing chamber filled with ink and having the individually addressable heating element resistor thermally coupled to the ink. As the bubble nucleates and expands, it displaces a volume of ink which is forced out of the nozzle and deposited on the medium. The bubble then collapses and the displaced volume of ink is replenished from a larger ink reservoir by way of ink feed channels.
After the deactivation of the heater resistor and the expulsion of ink from the firing chamber, ink flows back into the firing chamber to fill the volume vacated by the ink which was expelled. It is desirable to have the ink refill the chamber as quickly as possible, thereby enabling very rapid firing of the nozzles of the printhead.
The ink flow into the chamber is through an entrance channel. In some printheads, the entrance channel is narrowed at a pinch point, to control the flow rate, e.g. in cases where different ink channels have different lengths from the ink source. It is desirable in a typical printhead to provide relatively equal flow rates to all the firing chambers of the printhead, to provide good print quality. The pinch points are employed to aid in this goal.
Prolongation of printhead life is one goal of printhead designers. One failure mode for printheads, which leads to shortened life, is failure of the resistors due to damage resulting from firing the resistor. This problem is exacerbated when the printhead is designed to produce droplets of relatively high drop weight, typically 8 nanograms or larger, and relatively high firing rates, typically 12 Khz or greater.
One technique which has been employed with inkjet printheads to seek to reduce resistor damage is to move the nozzle bore, along a center line through the resistor and ink feed channel, toward the firing chamber back wall, to move the bubble collapse off the resistor into the ink feed channel.
A printhead apparatus and method has a plurality of ink drop generators coupled to a source of ink. Each ink drop generator includes an orifice with a corresponding ink firing chamber and a heating resistor, and an ink feed channel coupling the firing chamber to the source of ink. The geometry of the ink drop generator relative to the heating resistor is selected to introduce an asymmetry to create a rotational component to the ink fluid velocity during bubble collapse. This rotational component, in turn changes the location or intensity of the steam bubble, lessening the damage this collapse causes on the resistor, and thereby increasing the resistor life for the printhead.